Especially in the field of automation technology, both of factory—as well as also process automation technology, the ignition protection type of a device installed in an explosion endangered area plays an important role. The ignition protection type should exclude the risk that in an explosive, or explosion endangered, atmosphere an ignition source occurs, which can trigger an explosion. Ignition protection types used in measuring- and automation technology either block the explosive atmosphere, e.g. by shielding the device by encapsulation or a potting compound, or eliminate ignition sources.
An extremely important ignition protection type in automation technology, especially in the chemical and petrochemical industry, is the ignition protection type Ex i. Devices with ignition protection type Ex i are built to be intrinsically safe, i.e. by special principles of construction, it is assured that, even upon the occurrence of a defect, there is no danger of an explosion in an explosive atmosphere. In intrinsically safe devices, the supplied electrical current and the applied voltage and therewith, the supplied power, are sized sufficiently small that a spark formation, in the case of a switching or in the case of a short circuit, is not sufficient for igniting the explosive atmosphere. The advantages of intrinsically safe devices include that complex housing constructions become unnecessary and maintenance tasks can be performed even during ongoing operating.
It is a special feature of ignition protection type Ex i that, in such case, the Ex protection is, as a rule, implemented outside of the Ex region. Especially, the lines leading into the Ex region are made safe by so-called Ex barriers. The Ex barriers limit electrical current and voltage and are so dimensioned that the electrical circuit is not capable of igniting an explosive atmosphere and the downstream components to be made safe are supplied only with a suitably limited power.
In the case of intrinsically safe devices, it must, moreover, be assured that the surface temperature of the components coming in contact with the explosive atmosphere is limited to predetermined maximum values. For electronics, this means that the surface temperature of the individual components, or, when the electronics is potted, the surface temperature of the potted electronics, has to be limited. In order to satisfy this requirement, it has long been the practice to limit the power of individual components, or also of individual regions, into which a number of components can be combined. Moreover, adjoining components or regions are spaced from one another, wherein the separation between the components and/or regions is dimensioned such that mutual influencing is prevented. The limiting of surface temperature is assured by appropriately sized upstream- or limiting resistors.
The above described manner of limiting surface temperature has, in principle, the disadvantage that the power of individual components or regions must be strongly limited as a function of the upstream- or limiting resistors. Therefore, it is not possible in the case of ignition protection type Ex i to operate components with large upstream- or limiting resistors when the components need the high power.
In the case of known solutions, the same problem is present, when the intrinsically safe device, especially, thus, a field device, is operated in a clocked manner. Clocking means, in this connection, that the on-time is less than the period of the clocking. In the case of a component operated with clocking, an option is to take into consideration the average power consumption of the device for calculating the surface temperature. The average power PAV is calculated with the formula,PAV=Pmax×(on-time/period)+P* and is therewith regulatable via the ratio of on-time to period. In the formula, P* designates the sum of powers, which are transmitted via further connecting lines, such as signal-, control- and communication lines. The corresponding situation is illustrated in FIGS. 2a and 2b. In practice, the clocking thus provides the opportunity also to transmit very high amounts of energy into the Ex region, when the ratio of on-time and period is correspondingly limited.